In an engine starting device (hereinafter, referred to as a starter) of the related art, a starting operation is performed in a state where an engine is stopped. Accordingly, a pinion gear has been engaged with a ring gear, in a state where the ring gear is not rotating. However, in a system for performing idling stop for low fuel consumption, restarting performance is secured by engaging a pinion gear and a ring gear even in ring gear rotation.
For example, when restarting is required in a state where engine rotation is not stopped at the time of idling stop, or when shortening of time is required when restarting from the stopped state, engagement with a pinion gear is previously performed in rotation of a ring gear.
In this case, as a method of engaging a pinion gear in rotation of a ring gear, there has been known a method of engaging the gears by turning on and controlling the speed of a starter motor of the pinion gear so as to be synchronized with a rotation speed of the ring gear (for example, see Patent Literature 1). In addition, there has been known a method of engaging the gears by providing a mechanism for previous synchronization and performing synchronization to have constant difference of the rotation speed by friction of the mechanism unit (for example, see Patent Literature 2). Further, there has been known a method of devising a pinion formation to easily perform the engagement (for example, see Patent Literature 3).